Optical disks are widely used as large capacity information recording media. Technological development for increasing capacities of optical disks has proceeded from CDs to DVDs and then to Blu-ray Discs by adopting laser light of shorter wavelengths and objective lenses with higher numerical apertures (NA). Recently, given that services known as cloud services that utilize online storage on the Internet have been expanding year after year, a further capacity enlargement of storages including HDDs (hard disk drives) and flash memories is desired.
The following developments are underway with respect to further capacity enlargement of optical disks.
First, as far as wavelength reduction of laser light is concerned, a semiconductor laser that emits laser light in a 300 nm ultraviolet range has been put to practical use. However, since light in an ultraviolet range equal to or shorter than 300 nm attenuates significantly in air, reducing the wavelength of laser light cannot be expected to produce major benefits.
Next, as far as increasing NA is concerned, a technique has been developed for increasing recording surface density with a system that uses an SIL (solid immersion lens) having an NA of one or higher. In addition, research is being carried out with respect to increasing recording surface density through the use of a near-field light that occurs in a smaller region than a diffraction limit of light. Furthermore, while BD-XL among optical disks currently in the market has three or four recording surfaces, research aimed at enlarging capacity by further increasing the number of recording surface layers is also being conducted.
With the ongoing promotion of capacity enlargement of optical disks as described above, in particular, multi-layering causes a further reduction in signal light amount that is modulated due to reflection by a recording surface of an optical disk and prevents a sufficient S/N of a reproduction signal from being secured. Therefore, increasing an S/N of a detected signal should become essential in pursuing capacity enlargement of optical disks in the future.
Techniques for increasing an S/N of a reproduction signal of an optical disk include a detecting system that uses optical interference. With this detecting system, light from a laser is divided into signal light that is irradiated on an optical disk and reference light that is not irradiated on the optical disk, and interference is performed between reflected light (reproduced light) from the optical disk and the reference light. In addition, a weak signal amplitude of the signal light is amplified by increasing the amount of the reference light. While this technique is advantageous in that a weak reproduced light can be detected at a high S/N, since optical interference is used, a noise component is included in a reproduction signal when phases of the reference light and the reflected light from the optical disk fluctuate. Therefore, further creativeness is required. For example, solutions thereof are disclosed in Patent Literature 1 and Patent Literature 2.
FIG. 21 is a diagram showing a configuration of a conventional interferometric optical disk apparatus.
In Patent Literature 1, in order to stabilize a difference in optical path lengths between a reproduced light and reference light, a mirror drive unit 112 for adjusting an optical path length of the reference light is added to a reference light mirror 111 as shown in FIG. 21. Accordingly, control is performed so that a maximum signal amplitude is always obtained in response to a fluctuation in the optical path length due to camming during rotation of an optical disk 101 or in response to a variation over time in the optical path length due to temperature or the like.
In addition, Patent Literature 2 describes a system in which a corner-cube prism is used as a reference light mirror, the corner-cube prism is mounted on a same actuator as an objective lens, and an optical path length of light to be subjected to interference is adjusted according to an optical disk type or a recording layer to be read.
A decline in the S/N of an optical signal limits progress toward realizing an apparatus with a high transmission rate and high density in the optical disk field. In a similar manner, a high S/N is also necessary in order to realize a high transmission rate in the optical communication field or the optical interface (optical bus or optical USB (universal serial bus)) field. Even in the optical communication field and the optical interface field, in order to achieve a high transmission rate at low power, systems that transmit data by modulating a phase of light generated by a laser are becoming mainstream in place of systems that transmit data by modulating laser intensity. Therefore, techniques that provide optical phase control for accurately controlling a phase of light on a receiving side in order to remove optical phase fluctuation factors that occur on a communication path become important.
When detecting an optical phase using optical interference, an average relative relationship between a phase of signal light to be detected and a phase of light to be used for interference must be precisely controlled. If a constant relationship between an average phase of the signal light to be detected and a phase of the interfering light cannot be maintained, a detection sensitivity of a detected signal declines significantly. Therefore, controlling a phase relationship is extremely important for practical realization of this detecting system.
However, since the wavelength of light ranges from several μm to 400 nm and is extremely short, even a slight variation of an optical path length of interference light and an optical path length of signal light in the order of several ten nm can have a significant impact on signal detection sensitivity. This means that the optical path length of the interference light and the optical path length of the signal light must be kept constant at an accuracy of several ten nm. When a fluctuation factor exists on an optical path length, there is a problem that controlling the optical path length becomes extremely difficult. For example, in the case of an optical disk, an optical path length of a reproduced light from the optical disk varies significantly within a range of around 200 μm due to an effect of undulation of a recording surface of the optical disk. Patent Literature 1 discloses a technique which attempts to avoid this effect by integrating an optical system and having the optical system follow an undulation of a recording surface of an optical disk while correcting gradual fluctuations with an actuator.
However, with the configuration according to Patent Literature 1, when the optical disk tilts and an angle of the signal light varies, an optical path length of the signal light also varies. Tracking fluctuations of the optical path length at an accuracy of several ten nm with an actuator is extremely difficult and, in particular, fluctuations become totally uncontrollable at frequency bands that are higher than a certain level. Therefore, in order to put the technique described in Patent Literature 1 into practice, fluctuation due to undulation of a recording surface of an optical disk must be reduced to almost zero. Such fluctuations have been a major obstacle toward practical realization.
In addition, Patent Literature 2 discloses a configuration in which a corner-cube prism is mounted on an actuator of an objective lens and an optical path of reference light is varied by a same amount as an optical path of signal light. The issue of undulation of a recording surface of an optical disk causing a fluctuation in the optical path length of the reference light and the optical path length of the signal light also applies to the configuration according to Patent Literature 2, making practical realization extremely difficult.
Since the frequency of light is extremely high, an optical phase cannot be directly detected using modern technology. Therefore, an optical phase cannot be detected by any other method than detecting an optical phase using optical interference between signal light and reference light. However, since the wavelength of light used for optical communication, an optical bus, or an optical disk ranges from several μm to 400 nm and is extremely short, the technique described above (a phase detection technique by reference light using optical interference) needs to be a technique that accurately controls average phases of reference light and signal light at several tenths of the wavelength of light. While fluctuation factors of optical phases of reference light and signal light differ among an optical disk, optical communication, and an optical bus, optical phase control in the order of nanometers is required in any case.